Dear All
credit to the Global Phasing team who along with Phil Jeffrey spotted and supplied the fix for this nasty bug which was introduced in CCP4 7.0 update 039 (May 2017).
I am sorry to say this is one of the occasions when CCP4s internal testing has fallen well sort, and I apologise for the problems this will have caused. It looks like amplitudes with a I/sigma-<I>/sigma in the range 10-20 will have the wrong values. this will have made structure solution significantly more difficult. The Global Phasing team have been investigating the effects further.
Thanks to Clemens Vonrhein who e-mailed on the 8th, a fix went out in update 057 yesterday.
To clarify some questions I received in follow-up to my e-mail yesterday, this effects truncate, not ctruncate as used by default by ccp4i and ccp4i2. ctruncate is uneffected.
Charles